Global seagrass losses parallel significant declines observed in corals and mangroves over the past 50 years. These combined declines have resulted in accelerated global losses to ecosystem services in coastal waters. Seagrass meadows can be extensive (hundreds of square kilometers) and longlived (thousands of years), with the meadows persisting predominantly through vegetative (clonal) growth. They also invest a large amount of energy in sexual reproduction. In this article, we explore the role that sexual reproduction, pollen, and seed dispersal play in maintaining species distributions, genetic diversity, and connectivity among seagrass populations. We also address the relationship between long-distance dispersal, genetic connectivity, and the maintenance of genetic diversity that may enhance resilience to stresses associated with seagrass loss. Our reevaluation of seagrass dispersal and recruitment has altered our perception of the importance of long-distance dispersal and has revealed extensive dispersal at scales much larger than was previously thought possible.

A movement ecology framework is applied to enhance our understanding of the causes, mechanisms and consequences of movement in seagrasses: marine, clonal, flowering plants. Four life-history stages of seagrasses can move: pollen, sexual propagules, vegetative fragments and the spread of individuals through clonal growth. Movement occurs on the water surface, in the water column, on or in the sediment, via animal vectors and through spreading clones. A capacity for long-distance dispersal and demographic connectivity over multiple timeframes is the novel feature of the movement ecology of seagrasses with significant evolutionary and ecological consequences. The space–time movement footprint of different life-history stages varies. For example, the distance moved by reproductive propagules and vegetative expansion via clonal growth is similar, but the timescales range exponentially, from hours to months or centuries to millennia, respectively. Consequently, environmental factors and key traits that interact to influence movement also operate on vastly different spatial and temporal scales. Six key future research areas have been identified.

A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass-sediment-light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, noneutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass-sediment-light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass-sedimentlight feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass-sediment-light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

Two widely distributed seagrasses in Western Australia with contrasting dispersal strategies were studied in terms of their physical characteristics and morphology to understand how physical processes (wind, waves, and currents) drive dispersal. Posidonia australis releases floating fruit that contain a single negatively buoyant seed that lacks dormancy. Halophila ovalis produces fruit and dormant seeds that sit on the sediment surface. The floating stage of P. australis was assessed in situ by tracking the movement of the fruit directly on the ocean surface, together with drifter devices to differentiate between the transport induced by surface currents and wind. The dehiscence time of P. australis fruit was evaluated in seawater tanks, and the associated viability of the seeds was assessed by growth after dehiscence. The settling velocities of P. australis seeds and H. ovalis fruit and seeds were quantified in settling tubes with image-processing techniques to track the fall trajectories. The re-suspension thresholds of the seeds were calculated based on the critical bed shear stresses required to transport the seeds in a unidirectional flow flume. P. australis can travel long distances at the air-sea interface (, 55 km), due to wind alone, during its floating stage. The settling velocities of P. australis and H. ovalis seeds (w s 5 10.6 6 0.4 cm s 21 and 4.7 6 0.1 cm s 21 , respectively) and their re-suspension thresholds (t 5 107 6 4 mPa and 66 6 1 mPa, respectively) suggest that secondary movement is restricted, but likely to be produced by stronger wave-induced shear stress events.The rate of global seagrass loss has accelerated during the past several decades due to a range of human activities in the coastal zone (Waycott et al. 2009). It is therefore critical to understand the processes controlling temporal and spatial dynamics of existing seagrass populations, including their growth, stability, and resilience. Sexual reproduction is an essential part of the life cycle of seagrasses, not only for colonization as previously thought, but also for consolidation of existing meadows as identified by high genetic diversity found in old meadows (Jover et al. 2003;Kendrick et al. 2012). Yet surprisingly, we still know very little about how the physical mechanisms of seed dispersal influence the distribution and structure of seagrass meadows. Despite the importance of sexual reproduction to seagrass population dynamics, most seagrass studies on reproductive effort have focused on clonal growth rather than sexual input (Orth et al. 1994). Research focused on sexual reproduction has led us to understand the biology of seed dispersal but not the demographic consequences.Seagrasses have evolved several times in the past from different terrestrial ancestral lineages (Les et al. 1997). Thus, despite sharing a common ecological niche, there are many differences between seagrass species. Some seagrasses are short-lived, with fast growth and a high production of seeds (termed R strategists; Duarte et al. 2006). Ot...

BackgroundSeagrasses are clonal marine plants that form important biotic habitats in many tropical and temperate coastal ecosystems. While there is a reasonable understanding of the dynamics of asexual (vegetative) growth in seagrasses, sexual reproduction and the dispersal pathways of the seeds remain poorly studied. Here we address the potential for a predominantly clonal seagrass, P. australis, to disperse over long distances by movement of floating fruit via wind and surface currents within the coastal waters of Perth, Western Australia. We first simulated the dominant atmospheric and ocean forcing conditions that are known to disperse these seagrass seeds using a three-dimensional numerical ocean circulation model. Field observations obtained at 8 sites across the study area were used to validate the model performance over ~2 months in summer when buoyant P. australis fruit are released into the water column. P. australis fruit dispersal trajectories were then quantified throughout the region by incorporating key physical properties of the fruit within the transport model. The time taken for the floating fruit to release their seed (dehiscence) was incorporated into the model based on laboratory measurements, and was used to predict the settlement probability distributions across the model domain.ResultsThe results revealed that high rates of local and regional demographic connectivity among P. australis meadows are achieved via contemporary seed dispersal. Dispersal of seeds via floating fruit has the potential to regularly connect meadows at distances of 10s of kilometres (50% of seeds produced) and infrequently for meadows at distances 100 s km (3% of seeds produced).ConclusionsThe spatial patterns of seed dispersal were heavily influenced by atmospheric and oceanographic conditions, which generally drove a northward pattern of connectivity on a regional scale, but with geographical barriers influencing finer-scale connectivity pathways at some locations. Such levels of seed dispersal infer greater levels of ecological and genetic connectivity and suggest that seagrasses are not just strongly clonal.Electronic supplementary materialThe online version of this article (doi:10.1186/s40462-015-0034-9) contains supplementary material, which is available to authorized users.